Best bet is with a reamp box.
Here's a good price on a Radial. One could probably argue that sending out a really low line level signal might work, but given the mismatched impedance, it is not a good idea. If you plan on doing a lot of reamping, this box will serve you well. If you are looking for something with a few more features, I have a little labs redeye that I highly recommend.
+1
While technically you can "make it work" with a low level line out, there are some complexities at play.
A typical guitar outputs a signal level around .5-1V (Which is not far below the fairly standard +4dBu reference level). However, a guitar has s significantly higher source impedance (on the order of several hundred K ohms), whereas a typical line output has a source impedance well under 100 ohms.
Signal is always transmitted between devices as a *voltage*. But that voltage must be developed across the the input impedance of the destination by the source impedance of the source.
Without going into all the nitty gritty details of Ohms law, signal transmission is "easiest" when the input impedance is about 10x higher than the source impedance. This is called a bridging input.
In line level interconnects you usually have a source impedance < 200 Ohms, and an input impedance in the 10-20k ohm range.
Guitars, with their very high source impedance, have great difficulty driving line inputs to the proper voltage (that's why guitars "seem" to be low level when connected to a line input).
Guitar amps are therefore usually set up with a very, very high input impedance, usually on the order of 1M ohm.
A line output has absolutely no problem driving this 1M ohm input. However... the design of the circuitry in the amplifier assumes the source impedance is going to be on the order of several K Ohms (a guitar). And depending on the topology of the circuitry, this could potentially have ramifications in terms of frequency response.
That's not to say it will sound "bad". But the circuitry will not be operating under the same conditions it would if the source were a high impedance.
The only way to properly simulate thus high impedance source is with something like a reamping box.
But again, a line output *will work*, it may just not yield the same sound / tone you would expect.